1. Field of the Invention
The present invention relates to a photorefractive optical recording medium, in particular to an optical recording medium whose optical characteristics is suppressed from deteriorating.
2. Description of the Related Art
A photorefractive medium is known as one of optical recording media that can record the information with a far higher density than the conventional media according to magneto-optical recording or photo-thermal phase-change recording, such as optical disc. The photorefractive medium can record large capacity data such as high-density images by changing the refractive index in the recording layer of the medium. A mechanism is as follows. When electric charges are generated inside of the medium by irradiating the light, these electric charges are spatially separated. Then the spatially separated electric charges generate the electric field. The electric field changes the refractive index of the recording material in the recording medium. Information is recorded in accordance with the change in the refractive index. When the electric field that is formed in the medium is large, large change in the refractive index will be observed, resulting from Pockels effect. The photorefractive medium can directly record an optical interference pattern as a refractive index pattern. The application for the photoreflactive medium is a holographic data storage and optical processing element, etc.
As materials for the photorefractive medium, inorganic ferroelectric crystals are widely known. Furthermore, recently, photorefractive polymers have been developed. The photorefractive polymers have dielectric constants far smaller than those of the inorganic crystals and are expected to have larger performance indexes and speedier responses, and can be easily produced (for example, Japanese Patent Application Laid-Open Nos. 6-55901 and 6-175167). The photorefractive polymer is a complex material made of molecules that show each function of charge generation, charge transportation, charge trapping and electro-optical effect, and the characteristics thereof can be tuned by changing a combination of the constituent molecules according to the use conditions.
In the conventional photorefractive recording medium, ordinarily, a polling process for orienting molecules is applied to the medium, in order to enhance a refractive index modulation of the medium by a generated internal electric field. The polling process comprises orienting the non-linear optical material molecules that cause refractive index modulation of the medium, enables to enhance the refractive index modulation at the time of the formation of a spatial electric field due to optical recording. Accordingly, the sensitivity of the medium can be enhanced, owing to the polling process.
However, the molecular orientation generated due to the polling process may be relaxed owing to heat or the like, resulting in a smaller refractive index modulation and smaller diffraction efficiency. Moreover, when the recording is carried out without applying an electric field, since the irradiated light and temperature at the recording may move the oriented molecules and a degree of orientation becomes smaller, the diffraction efficiency becomes smaller and multiplicity becomes lower. Additionally, in the medium easy to cause molecular orientation relaxation like this, it is frequently observed that optical quality deteriorates with the storing time of the medium. This is considered that decomposition and separation may easily occur according to easiness to move of the contained molecules.